An Innovative Beam Halo Monitor system for the CMS experiment at the LHC: Design, Commissioning and First Beam Results

The Compact Muon Solenoid (CMS) is a multi-purpose experiment situated at the Large Hadron Collider (LHC). The CMS has the mandate of searching new physics and making precise measurements of the already known mechanisms by using data produced by collisions of high-energy particles. To ensure high quality physics data taking, it is important to monitor and ensure the quality of the colliding particle beams. This thesis presents the research and design, the integration and the first commissioning results of a novel Beam Halo Monitor (BHM) that was designed and built for the CMS experiment. The BHM provides an online, bunch-by-bunch measurement of background particles created by interactions of the proton beam with residual gas molecules in the vacuum chamber or with collimator material upstream of the CMS, separately for each beam. The system consists of two arrays of twenty direction-sensitive detectors that are distributed azimuthally around the outer forward shielding of the CMS experiment. Each detector is comprised of a cylindrical quartz Cherenkov radiator, optically coupled to a fast ultraviolet-sensitive photomultiplier tube from one end and painted black at the opposite end. The novelty of the system is its directional sensitivity, which combined with the fast timing response of the detector units of a few nanoseconds, allows for the bunch-by-bunch measurement of the flux of the background particles produced upstream of CMS and the suppression of the signals induced by collision products. Monte Carlo simulations were performed to estimate the background and collision products fluxes at the detector location and the radiation environment and to optimize the detector design. The performance of the prototype detectors was validated experimentally in test beams. The system was designed to sustain the high radiation levels and be fully operational until the end of the LHC lifetime, including the HL-LHC era. A special magnetic shielding was designed to protect the photodetectors from the fringe magnetic field present at the system location. The readout electronics are based on the CMS Hadronic Calorimeter Phase 1 upgrade electronics. The electronics record the time of arrival and the integrated charge over 25 ns of each detector unit signal and deliver bunch-by-bunch occupancy histograms. The BHM software then receives and normalizes the results and publishes the bunch-by-bunch background histogram and a normalized background number for each beam to the LHC and CMS every 23 s, corresponding to 2^18 LHC orbits. The BHM system was installed during the first LHC long shutdown (LS1) and was commissioned with the first beams of Run 2 of the LHC in 2015 and is now providing the LHC machine and CMS experts useful feedback on the beam quality and the machine settings